Instruction manual

DDF1  7
latches the direction indicating LED. The adjustable delay is used to calibrate
the LED direction indicator with the actual direction of the transmission.
CIRCUIT DESCRIPTION
Take a look at the schematic of the WA2EBY Doppler RDF on page 18. The
heart of the system is an 8 kHz clock oscillator built around a 555 timer, U4,
configured as an astable multivibrator. C26, R27, and R28, R29 determine
the multivibrator's oscillation frequency. R27 and R28 are series connected
to allow fine tuning the oscillation frequency to 8 kHz. It is important that the
clock frequency be exactly 8 kHz; I recommend that it be adjusted to
+/-250 Hz of that frequency for reasons that I'll discuss shortly. The 8 kHz
output of U4 provides the clock for 4 bit binary counter U7. The 3 bit binary
coded decimal (BCD) output of U7 is used to operate three synchronized
functions.
8 KHz Clock Binary Counter
Digital Filter
Low Pass
Filter
High Pass
Filter
Zero Crossing
Detector
Adjustable
Delay
1 of 16 Data
Selector
LED Compass Display
1 of 4 Data
Selector
FM Receiver
Latch
Antenna
Switcher
External
Speaker
Ant
AF Out
R 36
Calibrate
Figure 2 Block Diagram of the WA2EBY Doppler RDF System
FIGURE 2
WA2EBY Doppler DDF System